This section provides background information related to the present disclosure which is not necessarily prior art. Certain barehand or common potential methods of servicing live, or energized, alternating current (AC) power lines are generally known to specially-trained or skilled individuals within the electrical construction and maintenance industry. Generally, barehand and common potential maintenance methods permit maintenance on power lines to be more efficient because electrical power does not need to be shut off to, or routed around, the power line for which maintenance is to be performed. In one instance of performing maintenance on a high voltage AC power line, an aerial lift platform, such as a bucket truck, may be equipped with an insulated, extendable boom to insulate workers in the bucket from ground potential and thus any potential difference with a high voltage AC power line, with which the workers may be in common potential. In conjunction with barehand and common potential methods used on AC power lines, an AC meter may be used to monitor current that passes through the insulated, extendable boom. While using such a meter and method on an AC power line has proven satisfactory, because Direct Current (DC) high voltage and associated current behaves much differently, and an AC meter and techniques are not satisfactory for work on a DC high voltage power line, a new DC meter and method of using the DC meter are desired.
Electrical power systems comprise several insulating structures, for example outdoor insulators. Energized lines are supported from support structures such as poles or towers by means of outdoor insulators. Such insulators are made of dielectric material such as porcelain, glass or other suitable material. These insulators tend to deteriorate over a period of time. One of the main causes for insulator deterioration is dielectric contamination. Outdoor insulators are continuously exposed to the environment and contaminants such as salt, dust, sand and other industrial pollutants tend to deposit or build-up on the insulator surface as a dry layer. The dry contaminant layer becomes conductive under light wetting conditions such as light rain or morning dew thereby reducing the dielectric performance of the insulator. Since one end of the insulator is energized, and the other end is grounded, reduced dielectric performance results in current flowing through the insulator to the ground. This current is typically referred to as leakage current. When the contamination is severe, leakage current can reach unacceptably high levels. When the leakage current exceeds a highest permissible value for a particular voltage class, it may result in a condition referred to as flashover. Flashovers create high temperature electrical arcs which may endanger line personnel, cause power outages and damage equipment.
Measurement and analysis of leakage current flowing through an outdoor insulator may be used to determine insulator degradation and consequently predict a flashover condition. Typically, a peak or RMS value of the leakage current is determined. This value is then correlated with flashover voltages to predict flashover. In an attempt to prevent flashover, leakage current flowing through insulators is periodically measured and analyzed.
Other predominant insulating structures in an electrical power system include an aerial boom or other support structures such as scaffolding, ladders or lattice towers. These structures enable workers to reach the overhead energized lines for conducting barehand work on the energized lines. Such structures include electrically insulating sections, which ensure that there is no electrical path from the energized lines to ground. The insulating structures allow a worker to work directly on the energized lines. If the electrical resistance of such insulating structures breaks down due to factors stated above, a worker could experience electric shock and injury.
There are several methods for detecting flow of leakage current through such insulating structures. Some known methodologies involve de-energizing the transmission line prior to testing. The methodologies discussed herein are directed to detection under live-conditions. In other words, the transmission lines are energized and not de-energized prior or during detection.
In conventional high voltage alternating current (AC) power systems, leakage current through insulating structures may be measured using AC multimeters such as those made by Fluke™ and a variety of other manufacturers. Such AC multimeters may be operably coupled to an insulator through electrical leads for measuring leakage current flowing through the insulator.
In recent years, transmission of power using high voltage direct current (HVDC) technology has been accepted as an alternative to conventional AC power systems. Insulating structures used in HVDC power systems are also susceptible to the dielectric degradation outlined above. However, due to fundamental differences between alternating current and direct current (bi-directional vs. unidirectional respectively), AC current measuring devices used in AC power systems for detection of leakage current cannot be safely used in a HVDC system.
A scientific paper titled “Insulator Leakage Current Monitoring: Challenges For High Voltage Direct Current Transmission Lines” by M. Roman et al. articulates the differences between AC and direct current (DC) power systems. It also corroborates that there is no direct mapping between AC and DC leakage current measurement devices.
DC meters for the measurement of leakage currents in low power applications, for example under 6 kv, are known. The sampling rate of such DC meters may be typically in the range of 60 to 100 Hz.
During Applicant's attempts to measure leakage current in HVDC systems, Applicant observed that leakage current is a composite DC current comprising transient spikes or discharges. Such discharges are high in magnitude and may be best described as “short duration” or “momentary” or “very narrow” spikes. In other words, the discharges are high in magnitude but typically extremely short in duration. Typically, such spikes have been observed by Applicant to have duration of less than a microsecond to a few hundredths of a second depending upon the energy of the spike. The greater the amplitude and duration, higher the energy. High energy spikes that exist for hundredths of a second are dangerous and represent an immediate risk of flashover. For this reason the lower energy short duration spikes are most critical to detect as they provide a safer and early warning.
In Applicant's experience, as the spikes are momentary, conventional DC meters do not react to such spikes and the spikes are not registered. Applicant believes that in order to capture such momentary spikes, conventional DC meters would have to be modified so that they have significantly higher than conventional sampling rates, for example, at least 10,000 samples/second (10 KHz). In addition these recorded spikes would need to be cataloged, and displayed to a user in a meaningful and timely way.
Therefore, there is a need for a relatively simple and inexpensive DC leakage current detecting apparatus or meter, without the need for a very high sampling rate, may be used with several types of electrically insulating structures in HVDC systems to indicate accurately leakage current voltage spikes flowing through such structures and display detected leakage current in a meaningful and timely way to an operator or user.